The present invention relates to a gradient coil for an MRI (magnetic resonance imaging) apparatus, a method of manufacturing a gradient coil for an MRI apparatus, and an MRI apparatus, and more particularly to a gradient coil for an MRI apparatus having a low electric power loss and a low heat release in its shielding coil, a method of manufacturing such a gradient coil, and an MRI apparatus.
FIG. 11 illustrates an exemplary magnet assembly in a conventional MRI apparatus.
The magnet assembly 51 comprises yokes 20, a pair of opposing permanent magnets 1Mt and 1Mb attached to the yokes 20 for generating a static magnetic field, magnetic field conditioning plates 24 and 25 disposed on the opposing surfaces of the permanent magnets 1Mt and 1Mb, respectively, for improving homogeneity of the static magnetic field, upper and lower Z-axis main gradient coils 1Zt and 1Zb disposed on the opposing surfaces of the magnetic field conditioning plates 24 and 25, respectively, for generating a Z-axis gradient magnetic field, an upper Z-axis shielding coil 51Zts for preventing magnetic flux generated by the upper Z-axis main gradient coil 1Zt from affecting the magnetic field conditioning plate 24, and a lower Z-axis shielding coil 51Zbs for preventing magnetic flux generated by the lower Z-axis main gradient coil 1Zb from affecting the magnetic field conditioning plate 25.
A combination of the upper Z-axis shielding coil 51Zts, the upper Z-axis main gradient coil 1Zt, the lower Z-axis main gradient coil 1Zb and the lower Z-axis shielding coil 51Zbs constitutes a Z-axis gradient coil 51Z.
Although omitted in the drawings, X- and Y-axis gradient coils are also disposed on the opposing surfaces of the magnetic field conditioning plates 24 and 25.
FIG. 12 is a schematic perspective view of the Z-axis gradient coil 51Z.
Windings of the upper Z-axis shielding coil 51Zts are disposed corresponding to the entire winding area of the upper Z-axis main gradient coil 1Zt. However, the number of the windings of the upper Z-axis shielding coil 51Zts is less than that of the upper Z-axis main gradient coil 1Zt.
Although the number of windings of the conventional Z-axis shielding coils 51Zts and 51Zbs is less than that of the Z-axis main gradient coils 1Zt and 1Zb, it is significantly large because the windings of the Z-axis shielding coils 51Zts and 51Zbs are positioned corresponding to the entire winding area of the Z-axis main gradient coils 1Zt and 1Zb. Thus, the Z-axis shielding coils 51Zts and 51Zbs have a large electric power loss.
Specifically, the conventional gradient coil for the MRI apparatus gives rise to a problem of generating a large electric power loss, and hence a large heat release in its shielding coil.
It is an object of the present invention to provide a gradient coil for an MRI apparatus having a low electric power loss and a low heat release in its shielding coil, a method of manufacturing such a gradient coil, and an MRI apparatus.
In accordance with a first aspect of the present invention, there is provided a gradient coil for an MRI apparatus comprising a partially shielding coil interposed between a main gradient coil and a magnetic component, the partially shielding coil having its windings positioned only in a high winding density zone containing a portion in which a winding density of the main gradient coil is highest.
In the gradient coil for an MRI apparatus of the first aspect, the windings of the shielding coil are positioned corresponding only to a high winding density zone, rather than corresponding to the entire winding area, of the main gradient coil. (For this reason, the coil is called a xe2x80x9cpartially shielding coilxe2x80x9d.) Accordingly, the number of the windings can be reduced, thereby reducing the electric power loss and the heat release. In addition, since the shielding performance required can still be attained, magnetic flux generated by the main gradient coil can be prevented from affecting a magnetic component (such as a magnetic field conditioning plate), thereby avoiding an adverse effect of remanence in the magnetic component.
In accordance with a second aspect of the invention, there is provided a method of manufacturing a gradient coil for an MRI apparatus, comprising the steps of applying an image electric current method only to a high winding density zone containing a portion in which a winding density of a main gradient coil is highest and applying a boundary condition only to the same zone to determine the position of windings of a partially shielding coil having its windings positioned only in the high winding density zone.
In the method of manufacturing a gradient coil for an MRI apparatus of the second aspect, the image electric current method is applied only to a high winding density zone and the boundary condition is applied only to the same zone, rather than to the entire winding area of the main gradient coil. Accordingly, the windings of the shielding coil can be suitably positioned corresponding only to the high winding density zone. (For this reason, the coil is called a xe2x80x9cpartially shielding coilxe2x80x9d.)
In accordance with a third aspect of the invention, there is provided a method of manufacturing a gradient coil for an MRI apparatus, comprising the steps of defining an optimization plane between a partially shielding coil and a magnetic component lying near the partially shielding coil, the partially shielding coil having its windings positioned only in a high winding density zone containing a portion in which a winding density of a main gradient coil is highest, and optimizing the position of the windings of the partially shielding coil by a least squares technique so that a magnetic field is minimized within the optimization plane.
In the method of manufacturing a gradient coil for an MRI apparatus of the third aspect, the windings of the shielding coil are positioned only in a high winding density zone, rather than over the entire winding area, of the main gradient coil, and the position of the windings is optimized by a least squares technique. Accordingly, the windings of the shielding coil can be suitably positioned corresponding only to the high winding density zone. (For this reason, the coil is called a xe2x80x9cpartially shielding coilxe2x80x9d.)
In accordance with a fourth aspect of the invention, there is provided a method of manufacturing a gradient coil for an MRI apparatus, comprising the steps of applying an image electric current method only to a high winding density zone containing a portion in which a winding density of a main gradient coil is highest and applying a boundary condition only to the same zone to determine the position of windings of a partially shielding coil having its windings positioned only in the high winding density zone, and then defining an optimization plane between the partially shielding coil and a magnetic component lying near the partially shielding coil, and optimizing the position of the windings of the partially shielding coil by a least squares technique so that a magnetic field is minimized within the optimization plane.
In the method of manufacturing a gradient coil for an MRI apparatus of the fourth aspect, the image electric current method is applied only to a high winding density zone and the boundary condition is applied only to the same zone, rather than to the entire winding area of the main gradient coil. Accordingly, the windings of the shielding coil can be suitably positioned corresponding only to the high winding density zone. (For this reason, the coil is called a xe2x80x9cpartially shielding coilxe2x80x9d.) Moreover, the position of the windings of the partially shielding coil is optimized by a least squares technique. Accordingly, the windings of the shielding coil can be more suitably positioned.
In accordance with a fifth aspect of the invention, there is provided an MRI apparatus comprising a gradient coil including a main gradient coil, and a partially shielding coil having its windings positioned only in a high winding density zone containing a portion in which a winding density of the main gradient coil is highest.
In the MRI apparatus of the fifth aspect, the windings of the shielding coil are positioned corresponding only to a high winding density zone, rather than corresponding to the entire winding area of the main gradient coil. (For this reason, the coil is called a xe2x80x9cpartially shielding coilxe2x80x9d.) Accordingly, the number of windings of the shielding coil can be reduced, thereby reducing the electric power loss and the heat release. In addition, since the shielding performance required can still be attained, magnetic flux generated by the main gradient coil can be prevented from affecting a magnetic component (such as a magnetic field conditioning plate), thereby avoiding image quality degradation due to an adverse effect of remanence in the magnetic component.
Thus, according to the gradient coil for an MRI apparatus, the method of manufacturing a gradient coil for an MRI apparatus and the MRI apparatus of the present invention, the number of windings of the shielding coil can be reduced, thereby reducing the electric power loss and the heat release. In addition, since the shielding performance required can still be attained, magnetic flux generated by the main gradient coil can be prevented from affecting a magnetic component, thereby avoiding image quality degradation due to an adverse effect of remanence in the magnetic component.
Further objects and advantages of the present invention will be apparent from the following description of the preferred embodiments of the invention as illustrated in the accompanying drawings.